set_config sets the parameter
setting_name to new_value. If is_local is true, the
new value will only apply to the current transaction. If you want
the new value to apply for the current session, use false instead. The function corresponds to the SQL
command SET. An example:

The functions shown in Table
9-45 send control signals to other server processes. Use of
these functions is restricted to superusers.

Table 9-45. Server Signalling Functions

Name

Return Type

Description

pg_cancel_backend(pidint)

boolean

Cancel a backend's current query

pg_reload_conf()

boolean

Cause server processes to reload their configuration
files

pg_rotate_logfile()

boolean

Rotate server's log file

Each of these functions returns true
if successful and false otherwise.

pg_cancel_backend sends a query
cancel (SIGINT) signal to a
backend process identified by process ID. The process ID of an
active backend can be found from the procpid column in the pg_stat_activity view, or by listing the
postgres processes on the server with
ps.

pg_reload_conf sends a
SIGHUP signal to the postmaster, causing the configuration files
to be reloaded by all server processes.

pg_rotate_logfile signals the
log-file manager to switch to a new output file immediately. This
works only when redirect_stderr is used
for logging, since otherwise there is no log-file manager
subprocess.

The functions shown in Table
9-46 assist in making on-line backups. Use of these functions
is restricted to superusers.

Table 9-46. Backup Control Functions

Name

Return Type

Description

pg_start_backup(labeltext)

text

Set up for performing on-line backup

pg_stop_backup()

text

Finish performing on-line backup

pg_start_backup accepts a single
parameter which is an arbitrary user-defined label for the
backup. (Typically this would be the name under which the backup
dump file will be stored.) The function writes a backup label
file into the database cluster's data directory, and then returns
the backup's starting WAL offset as text. (The user need not pay
any attention to this result value, but it is provided in case it
is of use.)

pg_stop_backup removes the label
file created by pg_start_backup,
and instead creates a backup history file in the WAL archive
area. The history file includes the label given to pg_start_backup, the starting and ending WAL
offsets for the backup, and the starting and ending times of the
backup. The return value is the backup's ending WAL offset (which
again may be of little interest).

Number of bytes used to store a particular value
(possibly compressed)

pg_tablespace_size(oid)

bigint

Disk space used by the tablespace with the specified
OID

pg_tablespace_size(name)

bigint

Disk space used by the tablespace with the specified
name

pg_database_size(oid)

bigint

Disk space used by the database with the specified
OID

pg_database_size(name)

bigint

Disk space used by the database with the specified
name

pg_relation_size(oid)

bigint

Disk space used by the table or index with the
specified OID

pg_relation_size(text)

bigint

Disk space used by the table or index with the
specified name. The table name may be qualified with a
schema name

pg_total_relation_size(oid)

bigint

Total disk space used by the table with the specified
OID, including indexes and toasted data

pg_total_relation_size(text)

bigint

Total disk space used by the table with the specified
name, including indexes and toasted data. The table name
may be qualified with a schema name

pg_size_pretty(bigint)

text

Converts a size in bytes into a human-readable format
with size units

pg_column_size shows the space
used to store any individual data value.

pg_tablespace_size and
pg_database_size accept the OID or
name of a tablespace or database, and return the total disk space
used therein.

pg_relation_size accepts the OID
or name of a table, index or toast table, and returns the size in
bytes.

pg_total_relation_size accepts
the OID or name of a table or toast table, and returns the size
in bytes of the data and all associated indexes and toast
tables.

pg_size_pretty can be used to
format the result of one of the other functions in a
human-readable way, using kB, MB, GB or TB as appropriate.

The functions shown in Table 9-48
provide native file access to files on the machine hosting the
server. Only files within the database cluster directory and the
log_directory may be accessed. Use a
relative path for files within the cluster directory, and a path
matching the log_directory configuration
setting for log files. Use of these functions is restricted to
superusers.

Table 9-48. Generic File Access Functions

Name

Return Type

Description

pg_ls_dir(dirnametext)

setof text

List the contents of a directory

pg_read_file(filenametext,
offsetbigint, lengthbigint)

text

Return the contents of a text file

pg_stat_file(filenametext)

record

Return information about a file

pg_ls_dir returns all the names
in the specified directory, except the special entries
"." and
"..".

pg_read_file returns part of a
text file, starting at the given offset, returning at most length bytes (less if the end of file is reached
first). If offset is negative, it is
relative to the end of the file.

pg_stat_file returns a record
containing the file size, last accessed time stamp, last modified
time stamp, last file status change time stamp (Unix platforms
only), file creation timestamp (Windows only), and a boolean indicating if it is a directory. Typical
usages include: